Patients: We studied 100 patients infected with human immunodeficiency virus (HIV) who had had 119 episodes of pulmonary dysfunction within 60 days after CD4 lymphocyte determinations.

Measurements and Main Results: Circulating CD4 counts were less than 0.200 X 109 cells/L (200 cells/mm3) before 46 of 49 episodes of Pneumocystis pneumonia, 8 of 8 episodes of cytomegalovirus pneumonia, and 7 of 7 episodes and 19 of 21 episodes of infection with Cryptococcus neoformans and Mycobacterium avium-intracellulare, respectively. In contrast, circulating CD4 counts before episodes of non-specific interstitial pneumonia were quite variable: Of 41 episodes, 11 occurred when CD4 counts were greater than 0.200 X 109 cells/L. The percent of circulating lymphocytes that were CD4+ had a predictive value equal to that of CD4 counts. Serum p24 antigen levels had no predictive value.

Conclusions: Pneumocystis pneumonia, cytomegalovirus pneumonia, and pulmonary infection caused by C. neoformans or M. avium-intracellulare are unlikely to occur in HIV-infected patients who have had a CD4 count above 0.200 to 0.250 X 109 cells/L (200 to 250 cells/mm3) or a CD4 percent above 20% to 25% in the 60 days before pulmonary evaluation. Patients infected with HIV who have a CD4 count below 0.200 X 109 cells/L (or less than 20% CD4 cells) are especially likely to benefit from antipneumocystis prophylaxis.